The present invention relates to an ultrasound blood flowmeter of a correlation detection type for measuring speed of blood which flows almost perpendicularly to two ultrasound wave beams (sound-beams) formed by the ultrasound blood flowmeter. More particularly, the present invention relates to an ultrasound blood flowmeter of a correlation detection type (correlation detection type blood flowmeter).
Usually, a Doppler effect type blood flowmeter is used in measurement of the blood flow speed; however, the speed is merely a component in a sound-beam direction. In other words, a speed component perpendicular to the sound-beam cannot be measured by the Doppler effect type blood flowmeter, whereas the correlation detection type blood flowmeter can measure the speed of blood which flows perpendicularly to the sound-beam. The correlation detection type blood flowmeter was invented by the inventor of the present invention and its contents were disclosed in Japanese laid-open patent No. 58-71464 in 1983. Thus, the subject matter of the present invention relates to an improvement of the correlation detection type blood flowmeter. Therefore, before discussing the present invention, the prior art correlation detection type blood flowmeter will be explained with respect to FIGS. 1 and 2.
FIG. 1 is a schematic diagram illustrating a relation between sound-beams (B.sub.1 and B.sub.2) formed by a prior art correlation detection type blood flowmeter 101 (blood flowmeter 101) and a blood flow 301 or 302 in a blood vessel 311 or 312 (shown with dotted lines) located in a human body 300. The blood flowmeter 101 comprises an ultrasound transducer (transducer) TD put onto a body surface 303, wherein transducer TD forms dual sound-beams B.sub.1 and B.sub.2 for measuring the speed of blood flow 301 which flows almost perpendicularly to sound-beams B.sub.1 and B.sub.2, namely transducer TD sends pulsed or burst ultrasound waves (burst sound waves) into human body 300 along sound-beams B.sub.1 and B.sub.2 and receives echo signals reflected by substances located along sound-beams B.sub.1 and B.sub.2. Reference symbols SV.sub.11 and SV.sub.12 are referred to as sample volumes and they lie along sound-beams B.sub.1 and B.sub.2 respectively, and sample volumes SV.sub.11 and SV.sub.12 are equally positioned from transducer TD. The location of the flowing blood is determined by another ultrasound imaging means, and the positions of sample volumes SV.sub.11 and SV.sub.12 are adjusted so that blood flow 301 is caught by sample volumes SV.sub.11 and SV.sub.12.
FIG. 2 is an example of a block diagram of blood flowmeter 101. In the figure, reference numeral 13 is a ultrasound transducer which corresponds to transducer TD in FIG. 1. Transducer 13 is a multi-element array type ultrasound transducer which consists of an plurality of transducer elements and operates like a familiar phased array antenna of a radar system. Applying transducer 13 to blood flowmeter 101, blood flowmeter 101 may operate in two modes: a scanning mode (not illustrated) and a focusing mode. The former is used for providing ultrasound imagery, such as B-mode imagery, and the latter is used for measuring blood speed; the two modes can be easily changed by an electronic means. FIG. 2 shows a case of the focusing mode which forms two focused sound-beams B.sub.1 and B.sub.2 ; actually, a blood flowmeter according to the present invention can operate in combination with a unit for the scanning mode for imagery, but it is omitted in FIG. 2. A control unit 11 generates timing and sampling control signals. The timing signal is for controlling transducer 13 and a switching circuit 14, and the sampling control V.sub.S is for controlling sample-and-hold (S-H) circuits 22 and 23. Under the control of control unit 11, a drive unit 12 outputs driving pulses for driving transducer 13 to send burst sound waves out from the transducer elements of transducer 13, wherein the generated driving pulses are synchronized with the timing signals from control unit 11. The switching unit 14 is for switching transducer 13 so as to send the burst sound waves and to receive reflected sound waves under the control of control unit 11. When switching unit 14 is turned to T, as indicated in FIG. 2, the transducer elements send the sound wave bursts. After the burst sound waves are sent, switching unit 14 is turned to R, and then the transducer elements receive the reflected sound waves and convert them into electric echo signals (echo signals) respectively.
Echo signals transduced by transducer 13 are fed to amplifier elements of an amplifier 15 through switching unit 14 and fed to delay-line units 16 and 17. Delay-lines 16 and 17 correspond to sound-beams B.sub.1 and B.sub.2 respectively, each of which consists of delay-line elements which correspond to the transducer elements and compensate time differences of the received signals so that the echo signals received along sound-beams B.sub.1 and B.sub.2 can be simply added by adders 18 and 19 respectively. Thus, the technique for timing the relation between the control time of respective driving pulses and the delay-time of respective echo signals provides sound-beams B.sub.1 and B.sub.2. The technique is similar to phase array antenna technique of a radar system, namely the transducer elements are simultaneously driven, and the burst sound waves are transmitted to a rather broad area covering both sound-beams B.sub.1 and B.sub.2, while on the other hand, the receiving characteristics of the sound-beams B.sub.1 and B.sub.2 are made so as to be very sharp, respectively.
The echo signals added by adders 18 and 19 are fed to S-H circuits 22 and 23 through amplifiers 20 and 21 respectively. Each of S-H circuits 22 and 23 is of a conventional type, and echo signals respectively added by adders 18 and 19 are sampled at a sampling time t.sub.s1 by a sampling control signal V.sub.S fed from control unit 11 and held. Sampling time t.sub.s1 is determined by observing the location of the flowing blood, and sampling control signal V.sub.S is produced by a manual adjustment of control unit 11. The determination of sampling time t.sub.s1 is equal to the determination of the positions of sample volumes SV.sub.11 and SV.sub.12. FIG. 3 shows a waveform chart illustrating the mutual time relation among the burst sound waves, the added echo signals, the sampling control signals, and the S-H voltages. FIG. 3(a) is a train of the burst sound waves each of which bursts at time t.sub.0 having period T; FIGS. 3(b) and 3(c) show the added echo signals with respect to sound-beams B.sub.1 and B.sub.2 ; FIG. 3(d) shows a train of the sampling control signals each being generated at time t.sub.s1 counted from each time t.sub.0 ; and FIGS. 3(e) and 3(f) show S-H voltages V.sub.SH1 and V.sub.SH2 which correspond to sample volumes SV.sub.11 and SV.sub.12 respectively.
The S-H voltages V.sub.SH1 and V.sub.SH2 are fed to a time-difference detection circuit 24 which is for detecting a time difference between respective peak amplitude of S-H voltages V.sub.SH1 and V.sub.SH2 by cross correlation technique; FIG. 4 illustrates a relation of a cross correlation between the peak amplitude. As shown in FIG. 4, the amplitudes of S-H voltages V.sub.SH1 and V.sub.SH2 have respective peaks, which is due to the fact that blood has such a nature that it flows in a state of being gathered in small masses of red blood corpuscles. The amplitude variation depends on size of the respective mass. When blood flows almost perpendicularly to sound-beams B.sub.1 and B.sub.2 passing through sample volumes SV.sub.11 and SV.sub.12, the time-difference detection circuit 24 picks up peak amplitude P.sub.11 and P.sub.21 from S-H voltages V.sub.SH1 and V.sub.SH2 respectively by cross correlation and provides a voltage V.sub.td indicating the time difference between individual peak amplitudes P.sub.11 (P.sub.12, - - - , or P.sub.1n) and P.sub.21 (P.sub.22, - - - , or P.sub.2n), namely producing an output voltage called a time-difference voltage V.sub.td. The time-difference detection circuit 24 comprises a fixed delay-line, a variable delay-line, and an automatic signal coincidence circuit; the details of which have been disclosed in Japanese laid-open patent No. 58-71464 in 1983 as mentioned before. The time-difference voltage V.sub.td is fed to a speed calculating circuit 25 in which the speed of blood which flows almost perpendicularly to sound-beams B.sub.1 and B.sub.2 passing through sample volumes SV.sub.11 and SV.sub.12 is calculated. The calculation is performed by dividing a distance between sample volumes SV.sub.11 and SV.sub.12 by time difference t.sub.td ; the distance can be previously obtained by determining the distance between transducer 13 and sample volume SV.sub.11 or SV.sub.12 and the angle (radians) between the sound-beams B.sub.1 and B.sub.2.
Thus, the correlation detection type blood flowmeter can measure the speed of blood which flows almost perpendicularly to the sound-beam; this is a great advantage compared with the Doppler type blood flowmeter which can measure only the speed of the blood in the direction of the sound-beam. However, in the prior art correlation detection type blood flowmeter, the sample volume SV.sub.11 or SV.sub.12 has almost no range, namely an undersirably small capability exists to detect the blood mass along the directions of sound-beam. Thus it has been very hard to detect the flowing blood, it being hard to adjust sample volumes SV.sub.11 and SV.sub.12 so as to catch the flowing blood. Further, the case often arises that the blood flows aslant to the sound-beam, as shown by blood flow 302 in FIG. 1. This has been a problem of the prior art correlation detection type blood flowmeter. Furthermore, a signal reflected by a fixed substance such as a blood vessel disturbs the detection of echo signals reflected by the flowing blood, which has been another problem of the prior art correlation detection type blood flowmeter.